A known method of transmitting picture data is described in U.S. Pat. No. 4,829,569. In this known method the picture data is constituted by teletext pages. The pages are scrambled in such a way that they can only be received by the selected receiver.
Use of the known method is limited to situations in which the quantity of picture data to be transmitted fits within the capacity of the transmission medium. Problems arise if the capacity is inadequate to frequently provide a plurality of receivers with new picture data. An example of a transmission medium which does not have the required capacity is the mains. Nowadays, the mains is not only used for distributing electric energy, but also for remote control of street lighting, boilers and the like, and for remote reading of electricity meters. To control the consumption of electricity, an ever-increasing number of electricity producers resorts to remote reading of the electric power in individual houses and to adapting the tariffs or controlling a limiter circuit in dependence upon these tariffs. Such measures are expected to be acceptable and to function correctly only if the individual user is clearly informed about the actual state and the possible consequences of his consumer behaviour. However, it has been found that the channel bit rate of the mains is insufficient for providing each house often enough with the data for forming a picture representing the relevant information in a clear way.